Fuel cells have been used as a power source in many applications and have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton conductive, non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is typically referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluorinated sulfonic acid ionomers. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies, which comprise the catalyzed electrodes, require certain controlled conditions in order to maintain certain hydration for optimized proton conductivity and avoid flooding.
Efficient operation of a fuel cell depends, at least in part, on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of product water is desired so as to not inhibit flow of fresh reactants to the catalytic sites. Therefore, it is desirable to improve the mobility of reactant and product water to and from the MEA where reaction occurs.
To improve the mobility of reactant and product species to and from the MEA where reactions occur, a diffusion structure which enhances mass transport to and from an electrode in a MEA of a fuel cell is used. The diffusion structure cooperates and interacts with an electrode at a major surface of the electrode opposite the membrane electrolyte of the cell, therefore, electrical and heat conductivity are required. The diffusion structure facilitates the supply of reactant gas to the electrode. The diffusion structure is hereinafter referred to as a diffusion media. See for example U.S. Pat. No. 6,350,539 issued to the assignee of the present application. The diffusion media is positioned between the MEA and the cathode or anode flow channels of an individual fuel cell. One example of a relatively typical diffusion media comprises an electrically conductive porous media such as carbon paper.
In an operating PEM fuel cell, water is generated at the cathode side due to the electrochemical reaction between hydrogen and oxygen occurring within the MEA. Water is also typically introduced through reactant gas streams into fuel cells to humidify the membrane to ensure good proton conductivity. PEM fuel cells can experience a relative excess of water, which, if not removed from the system, could block the transportation path between oxidant gas and cathode electrode. In addition to possible oxidant starvation on the cathode side, water slugs in the gas flow channel may also be formed on the anode side which can cause hydrogen starvation. Water on the anode side can result from external humidification of the hydrogen gas and from back diffusion through the membrane (cathode to anode). If these occur, the fuel cell efficiency can decrease and may eventually lead to system shutdown, a phenomenon called “flooding.” Managing water is therefore a relatively important aspect for the efficient operation of a fuel cell.
The diffusion media plays a relatively important role in PEM fuel cells water management. The diffusion media can facilitate movement of water to ensure good transportation paths between reactant gases and catalyst electrodes in the MEA. One conventional practice to accomplish this is to coat the diffusion media (such as carbon paper) with a hydrophobic material such as polytetrafluoroethylene (PTEE). This PTEE coating makes the media more hydrophobic and thus helps to prevent water from blocking the flow channels in diffusion media. Even still, other water management properties are sought to provide more efficient water management. It would be desirable for the gas diffusion media to provide a flow path for increased water management in a fuel cell.